Alternating-current motor control apparatus



Nov. 13, 195] J, c, MQUZQN 2,574,837

ALTERNATING CURRENT MOTOR CONTROL APPARATUS Filed Feb. 26, 1949 4Sheets-Sheet 1 JNVENTOR. JAMES C. MOUZON ATTORNEY Nov. 13, 1951 J. c.MOUZON 2,574,837

ALTERNATING CURRENT MOTOR CONTROL APPARATUS Filed Feb. 26, 1949 4Sheets-Sheet 2 FIG. 2

HIGH SPEED LOW SPEED OPERATION OPERATION MOTOR DRIVE SIGNAL 0 ANooEVOLTAGE o 0 OF VALVE -3 FEED-BACK o SIGNAL VOLTAGE O VOLTAGE 0 TOTERMINAL B TERMINAL 0 CD1 VOLTAGE 0 I DUE To FEED BACK VOLTAGE-OVOLTAGE-0 JNVENTOR.

JAMES C. MOUZON ATTORNEY Nov. 13, 1951 J. c. MOUZON 2,574,837

ALTERNATING CURRENT MOTOR CONTROL APPARATUS Filed Feb. 26, 1949 4Sheets-Sheet 3 JNVENTOR. JAMES C. MOUZON ATTORNEY Nov. 13, 1951 J. c.MOUZON 2,574,837

ALTERNATING CURRENT MOTOR CONTROL APPARATUS Filed Feb. 26, 1949 4Sheets-Sheet 4 FIG. IO

INVENTOR. JAMES c. mouzom ATTORNEY.

Patented Nov. 7 13, 1951 ALTERNATING-CUBRENT MOTOR CONTROL APPARATUSJames C. Mouzon, Wyndmoor, Pa., assignor to Minneapolis-HoneywellRegulator Company, Minneapolis, Minn., a corporation of DelawareApplication February 26, 1949, Serial No. 78,568

9 Claims. 1

The present invention relates to a motor control system of a well knowntype now in general use in self-balancing measuring apparatus in whichthe controlled motor is employed to rebalance a measuring circuit whenthe latter is unbalanced by a change in the quantity or conditionmeasured. In that system, the motor is energized and controlled by meansof an electronic voltage amplifying and motor driving system on which isimpressed an alternating voltage signal varying in magnitude and phasewith the extent and direction of measuring circuit unbalance.

The general object of the present invention is to provide a motorcontrol system of the type mentioned with improved motor damping means.More specifically, the object of the present invention is to so combinemeans, responsive to the operating condition of the motor, with voltageamplifying and motor drive apparatus of the type mentioned so that themotor will be subjected to a damping action which is initiated in thecourse of a rebalancing operation when the extent of unbalance becomessmall, and is terminated when balance is restored, and which does notinterfere with or modify the operation of the motor when the unbalanceis relatively large.

More specifically, the object of the invention is to control the dampingaction by means of an electronic damping control valve connected to thevoltage amplifying and motor driving system, and operative to subjectthe motor to a damping action when balance is being approached and themotor drive current is reduced, but inoperative to produce a dampingeffect when the motor speed is either relatively large or is reduced tozero.

The various features of novelty which characterize my invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,however, its advantages, and specific objects attained with its use,reference should be had to the accompanying drawing and descriptivematter in which I have illustrated and described preferred embodimentsof the invention.

Of the drawing:

Fig. 1 is a diagram illustrating motor control apparatus including novelmotor damping means;

Fig. 2 comprises curves showing voltages in different parts of theapparatus under different operating conditions;

Fig. 3 illustrates the use of a damping control valve diiferent fromthat shown in Fig. 1; and

Figs. 4, 5, 6, 7, 8, 9, and are diagrams each illustrating a difierentmodification of the apparatus shown in Fig. 1.

In Fig. l, I have illustrated the use of one desirable form of thepresent invention in a motor control system of the type employed in awell known and widely used type of self-balancing potentiometer. Asdiagrammatically shown in Fig. 1, T and T are vacuum tubes or electronicvalves included in a multi-stage amplifier employed to amplify analternating voltage signal of a magnitude and phase dependent on theextent and direction of unbalance of a measuring circuit. The signalsuccessively amplified by the tubes T and T is impressed on each of thecontrol grids of two power amplifier tubes or valves T and T As shownand as is customary, the tubes T, T, T and T are triodes of commerciallyavailable types. The valves '1 and T have a common output circuitincluding the control winding CW of a rebalancing motor M. As shown, thewinding CW has one terminal connected to ground and has its secondterminal C connected by a conductor 1 to the midpoint of the secondarywinding 2 of a transformer 3. The anode terminal D of the valve T isconnected to one end of the secondary winding 2, and the anode terminalE of the valve T is connected to the second end of the secondary winding2. The primary winding 4 of the transformer 3 has its terminals 5connected across alternating current supply conductors L and Lcustomarily supply alternating current of conventional voltage andfrequency, for example, volts and 60 cycles per second.

The control signal impressed on the control grid terminal A of the valveT is of the same frequency as the voltage across the supply conductors Land L and is approximately in phase, or out of phase, with the voltageacross the supply conductors, depending on the direction of measuringcircuit unbalance. The control winding CW is tuned to resonance by acondenser 6 connected in parallel with the winding, and a motor powerwinding PW has its terminals connected across the supply conductors Land L in series with a suitable phase shifting condenser 1, which inconjunction with the winding PW forms a series resonant circuit. Therotor of the motor M is ordinarily of the squirrel cage type and rotatesin one direction or the other accordingly as the signal impressed on theterminal A of the control grid of the valve T is in phase, or 180 out ofphase, with the voltage across the supply conductors L and L Thecathodes of the motor drive valves T and 'I are connected a couplingcondenser l9 and toground through a'common cathode resistor 8. Theanodes oi the valves T and T' are connected through resistors 9 and 8',respectively, each to a source of unidirectional current. The anode ofthe valve T is coupled by a condenser Hi to the terminal A 01 thecontrol grid of the valve T, and theanode of the valve T is coupled by acondenser II to the terminal B of the connected control grids for thevalves T' and T The control grid terminals A, A and B are connected toground by g'rid resistors l2, l3 and I4, respectively, and the cathodesof the valves T and T are connected directly to ground. In respect tothe portion of the voltage amplifying andv motor driving systemincluding the valves T, T, T and T and the motor M with its windings CWand PW, and the circuit connections already described, the apparatusshown in Fig. 1 does not differ significantly from that included in awell known and widely used self-balancing potentiometer of the generaltype and form disclosed in the Wills Patent 2,423,540 of July 8, 1947.

In accordance with the present invention, a damping control valve T isarranged to inject a damping voltage into the voltage amplifying andmotor driving system when needed. A signal voltage proportional to thesignal voltage to be amplified by the valves T and T is impressed on thecontrol grid A of the valve T through a connection G, and the valve Tinjects a damping voltage into the amplifier and motor drive systemthrough a controlling or feed-back connection F. As shown in Fig. l, thecontrol signal transmitted by the connection G to the valve T is aportion of the voltage impressed on the terminal A of the control gridof valve T, and the damping voltage is transmitted from the valve Tthrough the feed-back circuit branch F to the common terminal B of thecontrol grids of the motor drive valves T and T As hereinafterexplained, the signal and feed-back circuit connections to the controlvalve T may be connected to the voltage amplifying and motor drivingcircuit at points other than the points A and B of Fig. 1, providingthat the signals at these other points are in suitable phase relation.

As shown, the signal circuit branch G includes a resistor l6 having oneend connected to ground and having its other end connected by a couplingcondenser l! to the control grid terminal A of the valve T. The controlgrid terminal A of the valve T has its free end connected to a slidercontact adjustable along the resistance ill to regulate the signalvoltage transmitted to the valve T The cathode of the control valve '1'is connected to ground by a resistor l8. The feed-back circuit branch Fhas one end connected to a slider con'-- tactadiustable' along theresistor l8, and includes a resistor IS.

The valve T is assumed to be a gas filled triode or thyratron which maybe of the commercially available 884 type. and is characterized byhaving a critical firing characteristic such that for any givengrid-cathode potential, a certain minimum plate-cathode potential isrequired to make the tube conductive. As hereinafter explained,

the valve T need not be a thyratron, but may be a pentode with sharpcut-ofland arranged as shown in Fig. 3.

Preparatory to a-description of the operation of the apparatus shown inFig. l, attention is di-' rooted to the voltage magnitude'a'nd phaserelations illustrated bythe curves included inFig. 2. 7

Those curves are arranged in two vertical columns. The curves Al,El,,Cl, Fl, CDl, and MI inthe left-hand column "pertain "to voltagesexisting in certain specified portions of the apparatus shown in Fig.1during periods in which the measurin ratus, corresponding to those towhich the first column curves Al, El, etc. respectively pertain,

during a period in which measuring circuit balance is being approached,and in which the motor drive signal is small and the motor is'coasting.Each of the curves Al, A2, El, E2, Cl, C2 and MI is shown as of sinewave form and ordinarily approximates that form. Each ofthe curvesillustrates a voltage existing between the respective point and groundthroughout one full cycle of the alternating current supply voltage.

The curves Al and A2 illustrate the motor signal voltage at the point A,and differ from one another only as a result of the fact that the A2curve voltage is lower than the Al curve voltage. In respect to itsgeneral form and the phase relation which it indicates, the curve Aldoes not diiier from a curve which might be drawn to illustrate thealternating current supply voltage.

The curves El and E2 illustrate the voltage impressed on the anodeterminal E of the valve T by the transformer secondary winding 2, and donot difier from one another in magnitude or otherwise. The curves Cl andC2 illustrate the component of the motor voltage at the terminal C whichis due to the motor drive signal at the point A and to the motorrotation. As the curves indicate, the voltage represented by the curveCl is greater than the voltage represented by the curve C2. As will beobserved, the phase of the voltage represented by each of the curves El,E2, Cl, C2 and Ml is displaced from the phase of the voltage illustratedby each of the curves Al and A2. The curves Fl and F2 illustrate thefeed-back signal voltage impressed on the control grid terminal Bthrough the connection F. Since for the condition producing the curve Flthe valve T is non-conductive, and the feed-back signal is equal to zerothroughout both halves of the corresponding supply voltage cycle, theentire curve Fl is in the form of a straight line. During the first halfcycle to which the curve F2 pertains, the valve T is non-conductive andthe feed-back signal is equal to zero, and the first half of the curveF2 is also a straight line. The curves CDl and CD2 illustrate thecomponent of the motor voltage at the terminal C which is due to thefeed-back voltage signal illustrated by the curves Fl and F2. The curveCDl is a straight line and thefirst half of the curve, CD2 is also astraight line, in consequence of the fact that the motorvoltagecomponent due to feed-back is zero throughout the cycle to whichthe curve CDl pertains, and during the first half'of the cycle to whichthe curve CD2 pertains.

The curve Ml represents the net, or resultant, motor drive voltage atthe terminal C of the winding CW. Thus the curve Ml-is the resultant ofthe curves Cl and CDl, and hence is equal to Cl since the voltagerepresented by the curve CDl ,is

equal to zero throughout the cycle to which that agecomponentrepresented by the curve CD2 is equalto zero during the firsthalf of the. cycle to which that curve pertains, the first half of thecurve M2 isidentical to the first half of the'curve C2. since thevoltage component CD2 due to the feed-back signal is greater inmagnitude than, and opposite in polarity to, the voltage componentillustrated by the curve C2 during the second half of the cycle to whichthe curves C2 and CD2 pertain, the resultant motor voltage at theterminal C during the second half of the cycle to which the curve M2pertains is negative relative to ground as is the motor voltagerepresented by the curve C2 during the first half of said cycle in whichthe voltage represented by the curve CD2 is zero.

During each period in which the rebalancing motor is operating atsubstantially full speed, the control valve T is non-conductive and nopotential drop across the resistance It is then produced, and nofeed-back voltage signal is then fed to the terminal B through theconnection F. The nonconductivity of the valve T during full speedoperation periods may be explained as follows: During the first half ofeach supply voltage cycle, the tube T is non-conductive and the currentthen flowing through the tube T results in a voltage drop in the windingCW which makes the potential of the terminal C negative relative to theground potential, and thus prevents the control valve T from thenbecoming conductive. During the second half of each supply voltagecycle, the potential of the point B is made sufliciently negative withrespect to the ground potential by the full speed motor drive signal toprevent each of the tubes T and T from then becoming conductive.Although the condenser 6 is then operative to discharge through thewinding CW and thereby make the potential of the terminal C positivewith respect to the ground potential, the tube T is not thereby madeconductive because the control signal then being transmitted to thecontrol grid of the valve T from the point A is too strongly negativewith respect to ground to permit the tube T to fire.

When the motor drive signal at the point A drops below a certain minimumvalue, however, the signal fed through the connection G to the controlgrid of the valve T will not swing that grid sufficiently negative withrespect to ground to prevent the valve T from firing during the secondhalf of each supply voltage cycle in which the terminal C swingspositive with respect to ground. ,The extent to which the motor drivesignal at the point A must be reduced, below its value required foroperation of the motor M at full speed, may be varied as conditions makedesirable by the adjustment along the resistor [6 of the slider contactconnected to the grid terminal A of the valve T Even though the motordrive signal at the point B may have been substantially reduced by thedecrease in measuring circuit unbalance, the voltage between theterminal C and ground will remain fairly high while the motor iscoasting, since the voltage across the control winding CW isproportional to the speed of motor rotation when the motor is merelycoasting.

As soon as the tube T becomes conductive, the current flows through thecathode resistor [8 of the valve and produces a potential drop therein.That voltage drop makes the slider contact engaging the resistor l8positive with respect to ground. In consequence, the positive feed-backvoltage, shown by the curve F2, is then impressed on the grid terminalB. That voltage does not affect the valve T since the anode terminal Dof the valve T is then negative. However, the positive feed-back voltagesignal impressed on the point B makes the valve '1' conductive. When thevalve '1 is thus made conductive, it causes a current pulse to flowthrough the motor control winding CW in the direction to make theterminal C negative relative to ground, as is indicated by the curveCD2.

In practice, the adjustment position of the slider contact, throughwhich the feed-back connection F engages the resistor i8, should be suchthat the feed-back voltage signal impressed on the point B will cause avoltage pulse to be impressed on the winding CW which exceeds the normalvoltage across the winding CW sufilciently to produce the resultantvoltage shown by the second half cycle portion of the curve M2. As willbe readily apparent, the motor control winding voltage com; dent,illustrated by the curve C2 and resulting from the then small motordrive signal, COII'iOiIlCS with the damping voltage CD2 to produce thenet motor control winding voltage M2. The latter tends to drive themotor in the direction opposite to that in which the motor is thencoasting, and the motor M is thus subjected to a damping or brakingaction as a result of the feed-back signal voltage impressed on thepoint B when the valve T is made conductive.

In consequence of the damping action and the then small value of themotor drive signal. the motor speed is quickly reduced to zero, and thevoltage developed across the winding CW is decreased so that thefeed-back voltage transmitted to the point B through the connection F iscorrespondingly decreased. Finally just before the motor comes to rest,the voltage developed by transformer action across the winding CW dropsto a value which is insuilicient to maintain the conductivity of thevalve T whereupon the feedback voltage signal transmitted to the point Bi substantially reduced t zero. Thus as the motor comes to a stop, thedamping action disappears, with the desirable result of preventing thedevelopment of an unnecessarily large dead spot. While the motor M is atrest, the voltage developed across the winding CW is insufilcient tocause the valve T to be conductive even though the transformer actionwithin the motor may cause that voltage to be as high as 30 volts.

The foregoing description of the operation of the apparatus shown inFig. 1 has been confined to the operating condition in which thedirection of measuring circuit unbalance requires the rebalancingrotation of the motor M to be in the clockwise direction. When thedirection of unbalance is reversed so that the required rebalancingrotation of the motor is in the counter-clockwise direction, theoperation of the apparatus is substantially unchanged except that themotor winding CW is then energized by motor drive valve T while thevalve 'I is rendered conductive by the damping signal produced when thevalve T is made conductive. The motor drive signal at the point A whichproduces the counter-clockwise rotation of the motor M is out of phasewith the motor drive signal which results in the clockwise motorrotation. The counter-clockwise motor drive signal is thus in phase withthe voltage between the anode terminal E and ground, and is 180 out ofphase with the anode voltage between the valve T and ground.

Thu when the motor is being driven counterclockwise at substantiallyfull speed, the valve T is conductive during the second half only ofeach cycle of the supply voltage, and the signal applied t the controlgrid of the control valve T from the point A then prevents the controlvalve T from being conductive during the first half or each cycle. Asbalance is approached and the motor drive signal at the point A issufilciently reduced, the control valve T becomes conductive during thefirst half of each cycle, and a positive damping voltage is fed back tothe point B during the first half of each cycle of the supply voltage,i. e. during the half cycle in which the valve T can be made conductive.In consequence, the rebalancing motor is then subjected to a dampingaction which quicklybrings the motor to rest.

As previously indicated, various changes may be made in the form of theinvention illustrated in Fig. 1, and each of Figs. 3-8 illustrates adifferent modification of the apparatus shown in Fig. 1. In themodification shown in Fig. 3, the thyratron or gas filled valve T ofFig. 1 is replaced by a sharp cut-oil, high vacuum, pentode valve T Asshown, the suppressor grid of the valve T is connected directly to thecathode of the valve, and the screen grid of the valve is connected toground by a by-pass condenser 20. In Fig. 3, the screen grid of thevalve is also connected to a variable source of direct voltage. Asdiagram- .matically shown, the variable voltage source is a battery 2|having its negative terminal connected to ground, and having itspositive and negative terminals connected by a slide wire resistor 22.The latter is engaged by an adjustable slider contact connected to thescreen grid of the valve. By adjustment of said slider contact along theresistor 22, the operation of the valve TQr'nay be so adjusted that theanode voltage supplied through a the conductor I 5 from themotor controlwinding CW will be insuflicient to make the pentode. con.-

ductive when the rebalancing motor is not rotatphase with the voltagebetween the upper; ter-' minal C of the motor control winding CW andground. This shift in thefl'phase of 1 the anode voltage is attended byappropriatechanges in the connections in the control signal and feedbacksignal connections associated with the valve T, which enable themodified apparatus to operate in the same general manner asthe Fig.1apparatus. In Fig. 4, the feed-back connection F connects the cathoderesistor I8 of the valve'T control grid of the valve '1 is connected bya connection GB to the control grid terminal B oi the valve T. As shown,the connection GB comprises a conductor connected at one end to thecontrol grid 01 the valve T, and connected at its opposite end to aslider contact adjustable along the grid resistor 14 connected to thegrid terminal B. This arrangement is desirable in some cases in whichthe magnitude or the voltage of the grid terminal B of the valve '1 ismore suitable for controlling the valve T than is that of the voltage ofthe control grid A or the valve T.

In the modification shown in Fig. 5, the signal and feed-backconnections GB and F of the valve T are both connected to the terminal Bof the control grid of the valve T2. There is no disadvantage in suchconnections, since when the terminal B is positive, the control windingterminal C connected to the anode of the valve T is always negativerelative to ground and prevents the valve T from becoming conductive.When the point B swings negative with respect to ground as a result 01'a substantial motor drive signal, the adjustable resistors i8 and M, ifsuitably adjusted, will cause the grid of the valve T to swing negativeto a greater extent than does the cathode of the valve T so that thevalve will remain non-conductive. However, when the motor drive signal Bhas dropped to a predeterminable minimum value, the point B will notswing sufliciently negative to prevent the valve T from firing when thevalue of the voltage transmitted to the anode of the valve T through theconductor I5 from the motor control winding correspondsto a speed ofmotor rotation higher than is. desirable. Although the point B may besomewhat negative with respect to ground at the instant that the tube Tfires, the feedback connection to the point B from the resistor 18 willthen cause the grid terminal B to swing sharply positive, and cause thecontrol grid of the valve '1 to swing positive also. The positive swingof the control grid thus produced is inconsequento the control gridterminal A of the-amplifier "from the voltages of the terminals'A and Bor tial, since the valve T is already conductive. The

valve T returns to its non-conductive condition during the succeedinghalt cycle in which the anode of the valve T swings negative withrespect to ground. v

In the arrangements oi Figs. 1, 3,4,'and5,itis desirable to include theresistor IS in the feed-back connectionFin order to insure that the netgridcathode signal applied to the damping valve T will always be oi!sufflcient magnitude and of the correct phase to properly control theconductivity of that valve; In'the following arrangements of Figs. 6through lfl, however, theFresistor i9 is usually not needed, inasmuch asthese arrangethe valves T and T 'respectively. In Fig.4, the

control signal connection GA tothe valve T com which connect the'controlgrid of the valve T. to a slider contact engaginga slide wire resistor24 ments include'other means :or permitting the attainment of thecorrect magnitude and phase of the control signal for the valve T. 1

which serves as a cathode resistor'iorthe valve f T. A grid resistorZl"connects the controlg'rid J.

of the valve T" to ground; Aswill beappa'rent'glj. the modificationillustrated inFig. ioijtheapiparatus shown in Fig. 1 willnotchangethefgen-. eral operation of that apparatus,1provided' the control signalconnection GA and the feed b'ack" connection F of Fig. tareproperlyadiustedtdcompensate for the fact that .the potentialdifierences' between ground and the terminal A 2% the cathode of thevalve T. are somewhat diiierent from the potential-difl'erencesbetweenjground,

and the terminals A and Boi Fig. 1.

,In 6, Ihave illustrated an arrangement which maybe found useful underconditions making it desirable to impress a signal voltage on thecontrol grid of the valve T, which is higher than any signal voltagedirectly. obtainable from-the gri'dterminal-B of the valve T InF'igI fijthe terminal 13 is connected to 1 ground throng hi the primary winding25 of a step -up 'transiormeriifit The latter has a secondary winding-zlwi ljterminal grounded Y directlyi andjjwi ,tor GC and aslidewireresistorit *Thecontrol grid ot'the vtuve :T is connected-Itothe resistor 2a through a slider Contact adjustable alongfthe i'resistor'28. *By suitably varying the ratio oigthe terminal connected to groundthrong rim sa s s M seu th one former 23, any desirable voltage can bedeveloped across the resistor 28 when a given motor drive signal isimpressed on the control grid B oi the valve T. The primary andsecondary windings oi the transformer 26 are so arranged that thevoltage transmitted from the resistor 28 to the controlgrid of the valveT of Fig. 6 may be of the same phase as the voltage of the motor drivesignal impressed on the control grid of the valve T". When desirable,the coupling transformer 26 of Fig. 6 may be utilized in such forms ofthe invention as are shown in Fig. 1 and in Fig. 4. With the Fig. 4arrangement, however, it is necessary to reverse the leads to one 01'the windings of the transformer 26 to obtain the necessary 180 phaseshift needed to compensate for the signal phase shift produced if theprimary winding of the transformer 26 is connected to the grid electrodeof one of the valves T and T instead of to the grid or cathode electrodeof the valve T.

Fig. 7 illustrates a modification of the teed-back arrangement shown inFig. 1. In Fig. 7, the slider contact engaging the resistor I8 isconnected to the control grid terminal B of the valve T by a conductorFA in series with the grid resistor l4. The condenser I9 included in thefeed-back conductor F, shown in the forms of the invention previouslydescribed, is omitted in Fig. 7. The arrangement shown in Fig. '7 has anadvantage over each of the arrangements previously described, in that noportion of the Fig. 7 resistor I8 is e1- Iectively connected in parallelwith the grid resistor connected to the control grid to which thedamping signal is directly fed by the feed-back connection FA. As willbe apparent, however, a. portion of the cathode resistor I8 iseffectively connected in parallel with the grid resistor I4 in Figs. 1,5 and 6, and in parallel with the grid resistor I3 in Fig. 4.

In Fig. '7, the portion of the resistor l3 below the slider contactconnecting the resistor to the feed-back connection FA is connected inseries, not in parallel, with the grid resistor l4. The arrangementshown in Fig. 7 permits the resistance of the resistor I8 to be as smallas is desirable, while the resistor l8 must have a relatively highresistance, when it has a portion efl'ectively connected'in parallelwith the associated grid resistor M or l3, to avoid short circuitingsaid grid resistor.

A high resistance value for the resistor I8 is definitely undesirable incases in which the resultant large voltage drop in the plate circuit ofthe valve T is undesirable.

In Fig. 8, I have illustrated a modification embodying thecharacteristic features oi! Figs. 5 and '7, in that the feed-backconnection FA 01' Fig. '7 is combined with a conductor GB which connectsthe control grid of the valve T to the grid resistor l4 of the valve Tas it does in Fig. 5.

In each of Figs. 1-8, the voltage amplifying and motor drive system isassumed to be of the type and form illustrated in the above mentionedPatent 2,423,540. Fig. 9 illustrates an embodiment of the invention inapparatus in which the voltage amplifying and motor drive system is of aknown form differing from that disclosed in said prior patent. In Fig. 9the triode valve T constitutes the third stage valve of a systemamplifying an alternating signal formed by converting a small directcurrent into an alternating current in a converter of known form whichmay or may not be that shown in said prior patent. The valve T may wellbe one of the pair of valves in a twin valve tube of the commercially0nd stage valve,'not shown, of the voltage amplifier. The junction ofthe resistor 33 and condenser 34 is connected to ground through a gridresistor 33.

A power amplifier motor driving valve T has its control grid connectedto the anode of the valve T through a coupling condenser 31 and aresistor 38. The control grid of the valve T is also connected to groundthrough a resistor 41. As shown, the valve T" is a beam power tetrode.which may be of the commercially available 6L6 type, and is suppliedwith anode voltage through a resistor 40 from a direct current supplyconductor 33 at a potential positive relative to ground. The cathode ofthe valve T" is connected to ground through a resistor 4i and acondenser 42 in parallel therewith. The screen grid of the valve T" isconnected to ground through a condenser 43 and is connected to thesupply conductor 39 by a resistor 44. As shown, the resistor 40 isconnected to the supply conductor 39 in parallel with the controlwinding CW and condenser 3 of a rebalancing motor MA. The latter may bea two phase reversible motor like the motor M of Fig. 1. The powerwinding PW of the motor MA is arranged for connection through condenseri to conductors L and IF supplying alternating current of the samefrequency as the signal voltage amplified. As diagrammatically shown inFig. 9, the conductor 39 is the positive output terminal of a full waverectifier 45 which has its negative output terminal 46 connected toground. Alternating current to be rectified is supplied to the rectifier45 by supply conductors L and L to which the terminals of the rectifier45 may be directly con nected.

The modification shown in Fig. 9 includes a valve T having a reed-backconnection F and a control signal connection including a conductor GCand transformer 26. These connections lead to the control grid terminalof the valve T substantially in the same manner as the feedback andcontrol grid connections of the valve T oi Fig. 6 are connected to thegrid terminal B of the valve T The overall operation of the apparatusshown in Fig. 9 is substantially the same as that of the apparatus shownin Fig. 1 when modified as illustrated in Fig. 6.

In Fig. 10, I have illustrated a modification of the apparatus of Fig. 1wherein the control and feed-back connections G and F, respectively, ofFig. 1 are interchanged at the terminals A and B of the amplifier.Specifically, in the apparatus of Fig. 1 the control connection Gconnects the control grid terminal A 01' the damping valve T to thecontrol grid terminal B of the valve T while the feed-back connection Fconnects the cathode resistor 18 of the valve T to the control gridterminal A of the valve T. With this arrangement, the signal voltageavailable for application to the control grid of the valve T will alwaysbe greater than that applied to the cathode of this valve, since thesignal potential at the terminal B will be higher than that at theterminal A, due to the amplification efamass? fected by the'valves T andT. Further, the application of the reed-back signal to the point A inthe Fig. arrangement will cause-a desirably high feed-back voltage toappear at the terminal C, again due to the amplification produced by thevalves T and T. Aside from these ditierences, the operation of theapparatus of Fig. 10 is identical to that of the Fig. l apparatus.

While in accordance with the provisions of the statutes, I haveillustrated and described the best forms of'embodiment of my inventionnow known to me, it will be apparent to those skilled in the art thatchanges may be made in the forms of the apparatus disclosed withoutdeparting from the spirit of my invention, as set forth in the appendedclaims, and that in some cases certain features of my invention may beused to advantage without a corresponding use of other features. 7

Having 'now described my invention, what I claim as new and desire tosecure by Letters Patent is: g h

1. In motor control apparatus of known type in which an alternatingcurrent signal is amplified in a multi-stage electronic amplifierhavcathode elements of electronic valve means and including in seriestherewith a resonant circuit consisting of aparallel connected condenserand the control winding'ol ,a reversible alternating '25 ing an outputcircuit including the anode and current motor which has also a powerwinding arranged for connection to, a source of alternating current ofthe same frequency as'said signal, thev improvement which comprises anelectronic damping valve having a control grid and havcuit or saiddamping valve, and a control con-' nection and a feed-backconnectionrespectively connecting said control grid'and said cathode resistor topoints in said amplifier at which the voltages oi the signal amplifiedby the apparatus arefof substantially the same'phase.

2. Apparatus as specified in claim 1, in which said electronic valvemeans comprises two triodes each having an anode connected to arespective one of the end terminals of a power transformer secondarywinding having also a center-tap con 5. Apparatus as specified in claim1, in which said damping valve is a sharp cut-ofl? pentode.

6. Apparatus as specified in claim 1, in which said control andfeed-back connections connect the control grid of said damping valve andsaid cathode resistor respectively to points in separate stages of saidamplifier at which the signals amplified by the apparatus are ofsubstantially the same phase.

7. Apparatus as specified in claim 1, in which said control andfeed-back connections connect both the control grid of said dampingvalve and said cathode resistor to the input circuit of a single one ofthe stages of said multi-stage amplifier.

8. Apparatus as specified in claim 1, in which said control connectionconnects the control grid of said damping valve to a point in one of thestages of said multi-stage amplifier, and in which said feed-backconnection connects said cathode resistor to a point in a stage of saidamplifier which is ahead of said one stage.

9. In motor control apparatus of known type in which an alternatingcurrent signal is amplified in a multi-stage voltage amplifier havingits tubes having their anodes connected one to one end and the other tothe second end of the secondary winding of a'powe'r transformer havinits primary winding arranged for connection to a source of alternatingcurrent having the same frequency as said signal, and having thecathodes or said power tubes connected to the midpoint of said secondarywinding through a resonant circuit including the control winding of analternating current motor which'has also a power winding arranged forconnection ,to said source of alternating current, the improvement whichcomprises a thyratron having a control grid, an anode connected to theanodes of, said power tubes through. said secondary winding, and acathode connected through a cathode resistor to said power tubecathodes, and a control connection and a feed-back connectionrespectively connecting said control grid of said thyratron and saidcathode resistor to points in said amplifier at whichthe voltages of thesignal amplinection, and in'which the anode of said damping. Y

valve and one terminalot said control winding l are connected to saidcenter-tap connection or said secondary winding. v a

3. Apparatus as specified in claim 1, in which said valve means consists.of a single tetrode.

4.Apparatus as'specified in claim 1, in which said damping valve is athyratron.

same phase.

fied bythe apparatus. are of substantially the Y JAMES c. MoUzoN.

. 'asFEsEN sf CI D The following references are of record in the flleofthis patent:'

I s'ra'rssra'rs'n'rs Number V I v Name]: 1 Date 2,367,746 Williams Jan.23, 1945

